1. Field of the Invention
This invention relates to antennas, and more particularly to antennas suitable for use on aircraft.
2. Description of the Prior Art
Antennas designed for use with aircraft radios and guidance equipment are commonly available in the form of a streamlined blade-like member which is attached to the outer surface of the aircraft. A useful antenna of this type is described in U.S. Pat. No. 3,220,006 to David W. Young and Harvey P. Bazar, Ground Plane VHF Antenna Comprising Blade-Type Dipole Configuration Obtained By Reflecting Monopole In Ground Plane. In this device an elongated slot passes entirely through the width of a metallic blade member, extending diagonally from the area of the leading edge and base of the blade upwardly toward the trailing edge. The slot contains a dielectric material, the exterior surface of which is flush with the exterior surface of the blade member. A two-conductor transmission line feeds a signal to be radiated to the dielectric material, while the base of the blade is mounted on a conductive ground plane which provides a ground reference for the antenna. This type of construction was found to have improved mechanical strength, corrosion resistance, radiating efficiency, aerodynamic characteristics and lightning protection over previously available aircraft antennas.
While the blade antenna described above operates satisfactorily, it is always desirable to reduce the size of the antenna as much as possible so as to reduce air drag and weight, both of which are highly important factors for high-speed aircraft. The above-described prior art antenna blade must of necessity be longer than the length of the radiating dielectric material which it carries, while the dielectric material in turn is disposed in a strip the length of which is governed by the desired frequency of operation. No reduction in the length of this type of antenna, with a corresponding reduction in weight and air drag is possible without increasing its resonant frequency.
Most aircraft have a requirement for broadcasting within three different frequency ranges: one frequency for oral communication with the airport tower, a second frequency for glide slope signals, and a third frequency for radar broadcasts required by the transponder and DME (Distance Measuring Equipment). In order to reduce the number of separate blade antennas required, attempts have been made in the past to place two strips of dielectric material on the same blade, with the dimensions of each strip selected so that they respond to excitation in different frequency ranges. While it has been possible to achieve broadcasts in two different frequency ranges in this manner, it has been found that if one of the antennas is capable of transmitting over a relatively broad frequency range, the other strip is restrained to a relatively narrow band of frequencies, typically about 10 MHz. A practical system capable of broadcasting from a blade antenna over two separate wide band frequency ranges has not been available.